Systems and methods for peak current adjustments in power conversion systems

ABSTRACT

System and method for regulating an output of a power conversion system. An example system controller includes a signal generator and a modulation and drive component. The signal generator is configured to receive at least a first signal indicating a magnitude of an input voltage received by a primary winding of a power conversion system and receive a second signal indicating a magnitude of a primary current flowing through the primary winding, and generate a third signal. The modulation and drive component is configured to receive at least the third signal, generate a drive signal based on at least information associated with the third signal, and output the drive signal to a switch to affect the primary current.

1. CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No.201210529679.4, filed Dec. 10, 2012, commonly assigned, incorporated byreference herein for all purposes.

Additionally, this application is related to U.S. patent applicationSer. Nos. 12/859,138, 13/052,869 and 13/215,028, incorporated byreference herein for all purposes. Moreover, this application is alsorelated to U.S. patent application Ser. No. 13/646,268, incorporated byreference herein for all purposes.

2. BACKGROUND OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides adjustments of peak current. Merelyby way of example, the invention has been applied to power conversionsystems. But it would be recognized that the invention has a muchbroader range of applicability.

Generally, a conventional power conversion system often uses atransformer to isolate the input voltage on the primary side and theoutput voltage on the secondary side. To regulate the output voltage,certain components, such as TL431 and an opto-coupler, can be used totransmit a feedback signal from the secondary side to a controller chipon the primary side. Alternatively, the output voltage on the secondaryside can be imaged to the primary side, so the output voltage iscontrolled by directly adjusting some parameters on the primary side.

FIG. 1( a) is a simplified diagram showing a conventional flyback powerconversion system with primary-side sensing and regulation. The powerconversion system 100 includes a primary winding 110, a secondarywinding 112, an auxiliary winding 114, a power switch 120, a currentsensing resistor 130, an equivalent resistor 140 for an output cable,resistors 150 and 152, and a rectifying diode 160. For example, thepower switch 120 is a bipolar junction transistor. In another example,the power switch 120 is a MOS transistor.

To regulate the output voltage within a predetermined range, informationrelated to the output voltage and the output loading often needs to beextracted. In the power conversion system 100, such information can beextracted through the auxiliary winding 114. When the power switch 120is turned on, the energy is stored in the secondary winding 112. Then,when the power switch 120 is turned off, the stored energy is releasedto the output terminal, and the voltage of the auxiliary winding 114maps the output voltage on the secondary side as shown below.

$\begin{matrix}{V_{FB} = {{\frac{R_{2}}{R_{1} + R_{2}} \times V_{aux}} = {k \times n \times \left( {V_{o} + V_{F} + {I_{o} \times R_{eq}}} \right)}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where V_(FB) represents a voltage at a node 154, and V_(aux) representsthe voltage of the auxiliary winding 114. R₁ and R₂ represent theresistance values of the resistors 150 and 152 respectively.Additionally, n represents a turns ratio between the auxiliary winding114 and the secondary winding 112. Specifically, n is equal to thenumber of turns of the auxiliary winding 114 divided by the number ofturns of the secondary winding 112. V_(o) and I_(o) represent the outputvoltage and the output current respectively. Moreover, V_(F) representsthe forward voltage of the rectifying diode 160, and R_(eq) representsthe resistance value of the equivalent resistor 140. Also, k representsa feedback coefficient as shown below:

$\begin{matrix}{k = \frac{R_{2}}{R_{1} + R_{2}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

FIG. 1( b) is a simplified diagram showing a conventional operationmechanism for the flyback power conversion system 100. As shown in FIG.1( b), the controller chip of the conversion system 100 uses asample-and-hold mechanism. When the demagnetization process on thesecondary side is almost completed and the current I_(sec) of thesecondary winding 112 almost becomes zero, the voltage V_(aux) of theauxiliary winding 114 is sampled at, for example, point A of FIG. 1( b).The sampled voltage value is usually held until the next voltagesampling is performed. Through a negative feedback loop, the sampledvoltage value can become equal to a reference voltage V_(ref).Therefore,

V _(FB) =V _(ref)  (Equation 3)

Combining Equations 1 and 3, the following can be obtained:

$\begin{matrix}{V_{o} = {\frac{V_{ref}}{k \times n} - V_{F} - {I_{o} \times R_{eq}}}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

Based on Equation 4, the output voltage decreases with the increasingoutput current.

But the power conversion system 100 often cannot provide effectiveresponse to output loading changes. Hence it is highly desirable toimprove the techniques of primary-side sensing and regulation.

3. BRIEF SUMMARY OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides adjustments of peak current. Merelyby way of example, the invention has been applied to power conversionsystems. But it would be recognized that the invention has a muchbroader range of applicability.

According to one embodiment, a system controller for regulating anoutput of a power conversion system includes a signal generator and amodulation and drive component. The signal generator is configured toreceive at least a first signal indicating a magnitude of an inputvoltage received by a primary winding of a power conversion system andreceive a second signal indicating a magnitude of a primary currentflowing through the primary winding, and generate a third signal. Themodulation and drive component is configured to receive at least thethird signal, generate a drive signal based on at least informationassociated with the third signal, and output the drive signal to aswitch to affect the primary current. The signal generator and themodulation and drive component are further configured to, if an outputvoltage of the power conversion system is constant and an output currentof the power conversion system falls within a first predetermined range,generate a modulation signal as the drive signal based on at leastinformation associated with the magnitude of the input voltage withouttaking into account the magnitude of the primary current flowing throughthe primary winding, and if the output voltage is constant and theoutput current falls within a second predetermined range, generate themodulation signal as the drive signal based on at least informationassociated with the magnitude of the primary current without taking intoaccount the magnitude of the input voltage.

According to another embodiment, a system controller for regulating anoutput of a power conversion system includes a signal generator and amodulation and drive component. The signal generator is configured toreceive at least a first signal indicating a magnitude of an inputvoltage received by a primary winding of a power conversion system,process information associated with the first signal, and generate asecond signal based on at least information associated with the firstsignal. The modulation and drive component is configured to receive atleast the second signal, generate a drive signal based on at leastinformation associated with the second signal, and output the drivesignal to a switch to affect a primary current flowing through theprimary winding. The signal generator and the modulation and drivecomponent are further configured to, if an output voltage of the powerconversion system is constant and an output current of the powerconversion system falls within a first predetermined range, generate apulse-width-modulation signal corresponding to a pulse width and amodulation frequency as the drive signal. The pulse width decreases ifthe input voltage increases and if the output voltage and the outputcurrent remain constant.

According to yet another embodiment, a system controller for regulatingan output of a power conversion system includes a signal generator, afirst comparator, a second comparator, and a modulation and drivecomponent. The signal generator is configured to receive at least afirst signal indicating a magnitude of an input voltage received by aprimary winding of a power conversion system, process informationassociated with the first signal, and generate a second signal based onat least information associated with the first signal. The firstcomparator is configured to receive the second signal and a third signalassociated with a feedback signal of the power conversion system andgenerate a first comparison signal based on at least informationassociated with the second signal and the third signal. The secondcomparator is configured to receive the second signal and a thresholdsignal and generate a second comparison signal based on at leastinformation associated with the second signal and the threshold signal.The modulation and drive component is configured to receive at least thefirst comparison signal and the second comparison signal, generate adrive signal based on at least information associated with the firstcomparison signal and the second comparison signal, and output the drivesignal to a switch to affect a primary current flowing through theprimary winding. The modulation and drive component is furtherconfigured to, if the third signal is larger than the threshold signalin magnitude, output the drive signal to close the switch if the secondsignal is smaller than the third signal, and if the threshold signal islarger than the third signal in magnitude, output the drive signal toclose the switch if the second signal is smaller than the thresholdsignal.

In one embodiment, a method for regulating an output of a powerconversion system includes receiving at least a first signal indicatinga magnitude of an input voltage received by a primary winding of a powerconversion system, receiving a second signal indicating a magnitude of aprimary current flowing through the primary winding, and processinginformation associated with the first signal and the second signal. Themethod further includes generating a third signal, receiving at leastthe third signal, and processing information associated with the thirdsignal. In addition, the method includes generating a drive signal basedon at least information associated with the third signal, and outputtingthe drive signal to a switch to affect the primary current. The processfor generating a drive signal based on at least information associatedwith the third signal includes, if an output voltage of the powerconversion system is constant and an output current of the powerconversion system falls within a first predetermined range, generating amodulation signal as the drive signal based on at least informationassociated with the magnitude of the input voltage without taking intoaccount the magnitude of the primary current flowing through the primarywinding, and if the output voltage is constant and the output currentfalls within a second predetermined range, generating the modulationsignal as the drive signal based on at least information associated withthe magnitude of the primary current without taking into account themagnitude of the input voltage.

In another embodiment, a method for regulating an output of a powerconversion system includes receiving at least a first signal indicatinga magnitude of an input voltage received by a primary winding of a powerconversion system, processing information associated with the firstsignal, and generating a second signal based on at least informationassociated with the first signal. The method further includes receivingat least the second signal, processing information associated with thesecond signal, generating a drive signal based on at least informationassociated with the second signal, and outputting the drive signal to aswitch to affect a primary current flowing through the primary winding.The process for generating a drive signal based on at least informationassociated with the second signal includes, if an output voltage of thepower conversion system is constant and an output current of the powerconversion system falls within a first predetermined range, generating apulse-width-modulation signal corresponding to a pulse width and amodulation frequency as the drive signal. The pulse width decreases ifthe input voltage increases and if the output voltage and the outputcurrent remain constant.

In yet another embodiment, a method for regulating an output of a powerconversion system includes receiving at least a first signal indicatinga magnitude of an input voltage received by a primary winding of a powerconversion system, processing information associated with the firstsignal, and generating a second signal based on at least informationassociated with the first signal. The method further includes receivingthe second signal and a third signal associated with a feedback signalof the power conversion system, processing information associated withthe second signal and the third signal, and generating a firstcomparison signal based on at least information associated with thesecond signal and the third signal. In addition, the method includesreceiving the second signal and a threshold signal, processinginformation associated with the second signal and the threshold signal,and generating a second comparison signal based on at least informationassociated with the second signal and the threshold signal. Moreover,the method includes receiving at least the first comparison signal andthe second comparison signal, processing information associated with thefirst comparison signal and the second comparison signal, generating adrive signal based on at least information associated with the firstcomparison signal and the second comparison signal, and outputting thedrive signal to a switch to affect a primary current flowing through theprimary winding. The process for outputting the drive signal to a switchto affect a primary current flowing through the primary windingincludes, if the third signal is larger than the threshold signal inmagnitude, outputting the drive signal to close the switch if the secondsignal is smaller than the third signal, and if the threshold signal islarger than the third signal in magnitude, outputting the drive signalto close the switch if the second signal is smaller than the thresholdsignal.

Depending upon embodiment, one or more benefits may be achieved. Thesebenefits and various additional objects, features and advantages of thepresent invention can be fully appreciated with reference to thedetailed description and accompanying drawings that follow.

4. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a simplified diagram showing a conventional flyback powerconversion system with primary-side sensing and regulation.

FIG. 1( b) is a simplified diagram showing a conventional operationmechanism for the flyback power conversion system as shown in FIG. 1(a).

FIGS. 2( a) and 2(b) are simplified diagrams showing switching frequencyand peak current as functions of output current of a power conversionsystem according to an embodiment of the present invention.

FIG. 3 is a simplified diagram showing a power conversion system thatadjusts switching frequency and peak current in response to outputcurrent according to an embodiment of the present invention.

FIG. 4( a) is a simplified diagram showing a power conversion systemthat adjusts switching frequency and peak current in response to outputcurrent according to another embodiment of the present invention.

FIG. 4( b) is a simplified diagram showing certain components of thepower conversion system as shown in FIG. 4( a) according to anembodiment of the present invention.

FIG. 4( c) is a simplified diagram showing a power conversion systemthat adjusts switching frequency and peak current in response to outputcurrent according to yet another embodiment of the present invention.

FIG. 5 is a simplified timing diagram for the power conversion system asshown in FIG. 4( a) according to an embodiment of the present invention.

FIG. 6 is a simplified diagram showing certain components of the powerconversion system including the voltage-mode component as shown in FIG.4( a) according to an embodiment of the present invention.

FIG. 7( a) is a simplified timing diagram for the voltage-mode componentas part of the power conversion system as shown in FIG. 4( a) under aparticular condition according to an embodiment of the presentinvention.

FIG. 7( b) is a simplified timing diagram for the voltage-mode componentas part of the power conversion system as shown in FIG. 4( a) underanother condition according to another embodiment of the presentinvention.

5. DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides adjustments of peak current. Merelyby way of example, the invention has been applied to power conversionsystems. But it would be recognized that the invention has a muchbroader range of applicability.

Referring to FIGS. 1( a) and 1(b), information about the output voltageof the power conversion system 100 often is sampled only once everyswitching period. The switching period is inversely proportional to theswitching frequency, which usually is set low at no load or light loadconditions to reduce power consumption. But the low switching frequencyoften leads to poor dynamic response for the power conversion system 100if the load changes from no load or light load to full load. Forexample, if the switching frequency is several hundred Hz at no load orlight load conditions, information about the output voltage of the powerconversion system 100 is sampled once every several msec. If the loadchanges from no load or light load to full load (e.g., the outputcurrent changing to 1 A at full load), the output voltage may drop belowan acceptable level, because the controller does not respond until thenext sampling is performed after, for example, several msec. One way tosolve this problem is to increase the switching frequency at no load orlight load conditions. But if the switching frequency is increased, thepeak current of the primary winding at no load or light load conditionsshould be limited such that the output voltage does not exceed anacceptable level.

If the switching frequency is further increased, the peak current of theprimary winding at no load or light load conditions should be furtherreduced to decrease the standby power consumption. In a conventionalcurrent-mode pulse-width-modulation (PWM)/pulse-frequency-modulation(PFM) flyback power conversion system (e.g., the system 100), theinformation associated with a primary current flowing through theprimary winding is often needed to generate a pulse signal (e.g., a PWMsignal or a PFM signal) to close (e.g., to turn on) or open (e.g., toturn off) a power switch (e.g., the switch 120) in order to affect thepower delivered to the output load. A leading edge blanking (LEB) pulseis usually used to chop off on-spikes which often appear every cycle atthe beginning of a current-sensing process. For example, the width of aleading edge blanking pulse is usually in the range of 250 ns to 350 ns.The blanking pulse width and the propagation delay of a controller oftendetermine a minimum duration of an on-time period within a switchingperiod associated with the power switch (e.g., the switch 120). Usually,such a minimum duration of the on-time period is larger than what isneeded to regulate the output voltage at no load or light loadconditions in some applications, especially when the line input voltagesare high.

FIGS. 2( a) and 2(b) are simplified diagrams showing switching frequencyand peak current as functions of output current of a power conversionsystem according to an embodiment of the present invention. Thesediagrams are merely examples, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. The waveform 202 representsthe switching frequency (e.g., F_(SW)) as a function of output current(e.g., I_(out)), and the waveform 204 represents the peak current (e.g.,I_(s) _(—) _(peak)) for the primary winding as a function of outputcurrent (e.g., I_(out)). For example, if the power conversion system isat no load conditions; if I_(out)=I₆, the power conversion system is atmaximum load conditions; and if I₅≦I_(out)<I₆, the power conversionsystem is at full load conditions. In another example,I₁≦I₂≦I₃≦I₄≦I₅≦I₆. In yet another example, if I₁≦I_(out)≦I₆, the powerconversion system operates in an output-voltage regulation mode, forexample, a constant-voltage (CV) mode. In yet another example, ifI_(out)>I₆, as the output power remains at a maximum power, the outputvoltage drops with the increasing output current, and the powerconversion system no longer operates in output-voltage regulation mode,e.g., the CV mode. In yet another example, if the power conversionsystem operates in the constant-voltage (CV) mode, the output voltage isregulated to be at a predetermined voltage value.

As shown in FIG. 2( a), the switching frequency (e.g., F_(SW)) keeps ata minimum frequency f_(min) and does not change with the output current(e.g., I_(out)) if I₁≦I_(out)≦I₂ according to one embodiment. Forexample, the switching frequency (e.g., F_(SW)) changes with the outputcurrent (e.g., I_(out)) at a slope S_(1f) if I₂≦I_(out)≦I₃. In anotherexample, the switching frequency (e.g., F_(SW)) increases from theminimum frequency f_(min) (e.g., at I₁) to a frequency f₁ (e.g., at I₃).In yet another example, the switching frequency (e.g., F_(SW)) changeswith the output current (e.g., I_(out)) at a slope S_(2f) ifI₃≦I_(out)≦I₅. In yet another example, the switching frequency (e.g.,F_(SW)) increases from the frequency f₁ (e.g., at I₃) to a frequency f₂(e.g., at I₅). In yet another example, the switching frequency (e.g.,F_(SW)) changes with the output current (e.g., I_(out)) at a slopeS_(3f) if I₅≦I_(out)≦I₆. In yet another example, the switching frequency(e.g., F_(SW)) increases from the frequency f₂ (e.g., at I₅) to amaximum frequency f_(max) (e.g., at I₆). In yet another example, theswitching frequency (e.g., F_(SW)) keeps at the maximum frequencyf_(max) and does not change with the output current (e.g., I_(out)) ifI_(out)>I₆. In yet another example, each of the slopes S_(1f), S_(2f)and S_(3f) is larger than zero. In yet another example, the slope S_(2f)is equal to the slope S_(3f).

As shown in FIG. 2( b), the peak current (e.g., I_(s) _(—) _(peak)) foreach switching period (e.g., T_(sw)) changes with the output current(e.g., I_(out)) at a slope S_(1P) if I₁≦I_(out)≦I₂ according to anotherembodiment. For example, the peak current (e.g., I_(s) _(—) _(peak)) foreach switching period (e.g., T_(sww)) changes with the output current(e.g., I_(out)) at a slope S_(2p) if I₂≦I_(out)≦I₄. In another example,the peak current (e.g., I_(s) _(—) _(peak)) for each switching period(e.g., T_(sw)) changes with the output current (e.g., I_(out)) at aslope S_(3p) if I₄≦I_(out)≦I₅. In yet another example, the peak current(e.g., I_(s) _(—) _(peak)) for each switching period (e.g., T_(sw))changes with the output current (e.g., I_(out)) at a slope S_(4p) ifI₅≦I_(out)≦I₆. In yet another example, the slopes S_(1p) and S_(3p) eachare larger than zero. In yet another example, the slopes S_(2p) andS_(4p) each are equal to or larger than zero. In yet another example,the peak current (e.g., I_(peak)) has a minimum value I_(s) _(—) _(min)(e.g., at I₁) and a maximum value I_(s) _(—) _(max) (e.g., at I₅).

According to yet another embodiment, the power conversion systemoperates with voltage-mode pulse-width modulation (VPWM) forI₁≦I_(out)≦I₂. For example, in the VPWM mode, the information associatedwith the primary current flowing through the primary winding current isnot needed for generating a pulse signal (e.g., a PWM signal) to close(e.g., to turn on) or open (e.g., to turn off) a power switch. Inanother example, the leading edge blanking is not necessary for the VPWMmode, and thus the duration of an on-time period within a switchingperiod associated with a power switch is not limited by a blanking timeduration. In some embodiments, the power conversion system changes tocurrent-mode modulation (e.g., pulse-width modulation or pulse-frequencymodulation) for I₂≦I_(out)≦I₆. For example, the power conversion systemoperates with pulse-frequency modulation (PFM) for I₂≦I_(out)≦I₄. Inanother example, the power conversion system operates with bothpulse-frequency modulation and pulse-width modulation for I₄≦I_(out)≦I₅.In yet another example, the power conversion system operates withpulse-frequency modulation for I₅≦I_(out)≦I₆.

As shown above and further emphasized here, FIGS. 2( a) and 2(b) aremerely examples, which should not unduly limit the scope of the claims.One of ordinary skill in the art would recognize many variations,alternatives, and modifications. For example, the switching frequency(e.g., F_(SW)) changes with the output current (e.g., I_(out)) at aslope S_(4f) if I₅≦I_(out)≦I_(m), and changes with the output current(e.g., I_(out)) at a slope S_(5f) if I_(m)≦I_(out)≦I₆, whereI₅≦I_(m)≦I₆, and S_(4f) and S_(5f) are different.

FIG. 3 is a simplified diagram showing a power conversion system thatadjusts switching frequency and peak current in response to outputcurrent according to an embodiment of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. The power conversion system300 includes a system controller 302, a primary winding 360, a secondarywinding 362, an auxiliary winding 364, a switch 366, a current sensingresistor 368, an equivalent resistor 374 for an output cable, resistors370 and 372, rectifying diodes 376 and 382, and capacitors 378 and 380.The system controller 302 includes a voltage-mode component 304, a modecontroller 306, a frequency component 308, a sampling switch 310, anerror amplifier 320, a capacitor 322, a sampling controller 324, ademagnetization detector 326, an oscillator 328, a line sensingcomponent 330, resistors 332 and 334, a signal conditioning component336, a modulation component 338, a logic component 340, a drivingcomponent 342, comparators 344, 346 and 348, and a LEB component 350. Inaddition, the system controller 302 includes terminals 311, 312, 314,316 and 318. For example, the switch 366 is a transistor. In certainembodiments, the signal conditioning component 336 is omitted. In someembodiments, the LEB component 350 is omitted.

According to one embodiment, information about an output voltage 402 isextracted through the auxiliary winding 364. For example, the auxiliarywinding 364, together with the resistors 370 and 372, generates afeedback signal 404. In another example, the system controller 302receives the feedback signal 404 at the terminal 311 (e.g., terminalFB). When the switch 366 is opened (e.g., being turned off), the energystored in the transformer including the primary winding 360 and thesecondary winding 362 is released to the output terminal in certainembodiments. For example, the demagnetization process associated withthe transformer starts, and a secondary current 494 flowing through thesecondary winding 362 decreases in magnitude (e.g., linearly). Inanother example, when the demagnetization process almost ends and thesecondary current 494 flowing through the secondary winding 362approaches zero, the sampling controller 324 outputs a sampling signal498 to close (e.g., to turn on) the sampling switch 310 to sample thefeedback signal 404. In yet another example, after the sampling processis completed, the sampling controller 324 changes the sampling signal498 to open (e.g., to turn off) the switch 310. In yet another example,the sampled signal is held on the capacitor 322. In yet another example,a sampled and held signal 420 is generated at the capacitor 322 andreceived by the error amplifier 320 (e.g., at an inverting terminal). Inyet another example, the error amplifier 320 also receives a referencesignal 406 (e.g., V_(ref)) and generates an amplified signal 408 whichis associated with a difference between the signal 420 and the referencesignal 406.

The amplified signal 408 is used for selecting an operation mode (e.g.,by the mode controller 306), for adjusting switching frequency (e.g., bythe frequency component 308), and for affecting peak values of a primarycurrent 422 that flows through the primary winding 360 so as to affectthe power delivered to the output, in some embodiments. For example, theamplified signal 408 is received by the mode controller 306 whichgenerates a signal 428. In another example, the frequency component 308receives the signal 428 and outputs a signal 430 to the logic component340 which generates a signal 432. In yet another example, the drivingcomponent receives the signal 432 and generates a driving signal 499 toaffect the status of the switch 314. In yet another example, theamplified signal 408 indicates the output load conditions in closed loopregulation. In yet another example, the waveform of the driving signal499 is substantially the same as the waveform of the signal 432.

According to another embodiment, the primary current 422 that flowsthrough the primary winding 360 is sensed by the current sensingresistor 368, which in response outputs a current sensing signal 410 tothe comparators 344, 346 and 348 (e.g., through the LEB component 350).For example, if the switch 366 is closed (e.g., being turned on), thetransformer stores energy and the primary current 422 increases inmagnitude (e.g., linearly), causing the current sensing signal 410(e.g., V_(cs)) to also increase in magnitude (e.g., linearly). Inanother example, the comparator 346 also receives a signal 412 which isgenerated by the signal conditioning component 336 and associated withthe amplified signal 408, and outputs a comparison signal 436 to themodulation component 338. In yet another example, the comparator 344also receives a threshold signal 416 (e.g., V_(th) _(—) _(max)) andoutputs a comparison signal 438 to the modulation component 338. In yetanother example, the comparator 348 also receives another thresholdsignal 418 (e.g., V_(th) _(—) _(min) which is smaller than V_(th) _(—)_(max) in magnitude) and outputs a comparison signal 440 to themodulation component 338.

According to yet another embodiment, the feedback signal 404 is receivedby at least the demagnetization detector 326 and the oscillator 328. Forexample, the demagnetization detector 326 outputs a detection signal423, and the oscillator 328 also outputs a clock signal 424. In anotherexample, the line sensing component 330 outputs a signal 426 which isassociated with an input signal 442 (e.g., V_(in)). In yet anotherexample, the voltage-mode component 304 receives the signals 412, 426and 432 and outputs a signal 444 which is received by the modulationcomponent 338. In yet another example, the modulation component 338outputs a modulation signal 446 to the logic component 340 which outputsthe signal 432 to close (e.g., to turn on) or to open (e.g., to turnoff) the switch 366 in order to affect the primary current 422. In yetanother example, the signal 426 is proportional to the signal 442 inmagnitude, as follows.

If I _(—) vin≧0,I _(—) vin=α×V _(in)−β  (Equation 5)

where I_vin represents the signal 426, V_(in) represents the signal 442,and α and β represent constants respectively.

As discussed above and further emphasized here, FIG. 3 is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. For example, the switch 366 is a bipolar junctiontransistor (BJT), and the driving component 342 generates a currentsignal (e.g., the signal 499) to drive the switch 366. In anotherexample, the switch 366 is a metal-oxide-semiconductor field-effecttransistor (MOSFET) or an insulated-gate bipolar transistor (IGBT), andthe driving component 342 generates a voltage signal (e.g., the signal499) to drive the switch 366.

FIG. 4( a) is a simplified diagram showing a power conversion systemthat adjusts switching frequency and peak current in response to outputcurrent according to another embodiment of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. The power conversion system500 includes a system controller 502, a primary winding 560, a secondarywinding 562, an auxiliary winding 564, a switch 566, a current sensingresistor 568, an equivalent resistor 574 for an output cable, resistors570 and 572, rectifying diodes 576 and 582, and capacitors 578 and 580.The system controller 502 includes a voltage-mode component 504,comparators 506, 544, 546 and 548, a exponential generator 508, asampling switch 510, an error amplifier 520, a capacitor 522, a samplingcontroller 524, a demagnetization detector 526, an oscillator 528, aline sensing component 530, resistors 532 and 534, a signal conditioningcomponent 536, a modulation component 538, a logic component 540, adriving component 542, and a LEB component 550. In addition, the systemcontroller 502 includes terminals 511, 512, 514, 516 and 518. Forexample, the switch 566 is a transistor (e.g., a bipolar junctiontransistor). In certain embodiments, the signal conditioning component536 is omitted. In some embodiments, the LEB component 550 is omitted ifthe power conversion system 500 operates with a voltage-mode controlmode.

For example, the power conversion system 500 is the same as the powerconversion system 300. In another example, the system controller 502 isthe same as the system controller 302. In yet another example, theprimary winding 560, the secondary winding 562, the auxiliary winding564, the switch 566, the current sensing resistor 568, the equivalentresistor 574, the resistors 570 and 572, the rectifying diodes 576 and582, the capacitors 578 and 580, the voltage-mode component 504, thecomparators 544, 546 and 548, the sampling switch 510, the erroramplifier 520, the capacitor 522, the sampling controller 524, thedemagnetization detector 526, the oscillator 528, the line sensingcomponent 530, the resistors 532 and 534, the signal conditioningcomponent 536, the modulation component 538, the logic component 540,the driving component 542, and the LEB component 550 are the same as theprimary winding 360, the secondary winding 362, the auxiliary winding364, the switch 366, the current sensing resistor 368, the equivalentresistor 374, the resistors 370 and 372, the rectifying diodes 376 and382, the capacitors 378 and 380, the voltage-mode component 304, thecomparators 344, 346 and 348, the sampling switch 310, the erroramplifier 320, the capacitor 322, the sampling controller 324, thedemagnetization detector 326, the oscillator 328, the line sensingcomponent 330, the resistors 332 and 334, the signal conditioningcomponent 336, the modulation component 338, the logic component 340,the driving component 342, and the LEB component 350, respectively. Inyet another example, the terminals 311, 312, 314, 316 and 318 are thesame as the terminals 511, 512, 514, 516 and 518, respectively. In yetanother example, the exponential generator 508 and the comparator 506are part of the mode controller 306 and the frequency component 308 asshown in FIG. 3, and the mode controller 306 and the frequency component308 include one or more additional components.

According to one embodiment, information about an output voltage 602 isextracted through the auxiliary winding 564. For example, the auxiliarywinding 564, together with the resistors 570 and 572, generates afeedback signal 604. In another example, the system controller 502receives the feedback signal 604 at the terminal 511 (e.g., terminalFB). When the switch 566 is opened (e.g., being turned off), the energystored in the transformer including the primary winding 560 and thesecondary winding 562 is released to the output terminal in certainembodiments. For example, the demagnetization process associated withthe transformer starts, and a secondary current 694 flowing through thesecondary winding 562 decreases in magnitude (e.g., linearly). Inanother example, when the demagnetization process almost ends and thesecondary current 694 flowing through the secondary winding 562approaches zero, the sampling controller 524 outputs a sampling signal698 to close (e.g., to turn on) the sampling switch 510 to sample thefeedback signal 604. In yet another example, after the sampling processis completed, the sampling controller 524 changes the sampling signal698 to open (e.g., to turn off) the switch 510. In yet another example,the sampled signal is held on the capacitor 522. In yet another example,a sampled and held signal 620 is generated at the capacitor 522 andreceived by the error amplifier 520 (e.g., at an inverting terminal). Inyet another example, the error amplifier 520 also receives a referencesignal 606 (e.g., V_(ref)) and generates an amplified signal 608 whichis associated with a difference between the signal 620 and the referencesignal 606. The amplified signal 608 is used for adjusting switchingfrequency and for affecting peak values of a primary current 630 thatflows through the primary winding 560 so as to affect the powerdelivered to the output, in some embodiments.

According to another embodiment, the feedback signal 604 is received byat least the demagnetization detector 526 and the oscillator 528. Forexample, the exponential generator 508 receives a detection signal 622from the demagnetization detector 526 and a clock signal 624 from theoscillator 528, and outputs a signal 680 (e.g., V_(ramp)) to thecomparator 506. In another example, the comparator 506 also receives theamplified signal 608 and outputs a comparison signal 628 to the logiccomponent 540 in order to affect the switching frequency. In yet anotherexample, the logic component 540 generates a signal 632 to the drivingcomponent 542 which outputs a signal 699 in order to close (e.g., toturn on) or open (e.g., to turn off) the switch 566. In yet anotherexample, the signal 680 (e.g., V_(ramp)) is an exponential signal. Inyet another example, the waveform of the signal 699 is substantially thesame as the waveform of the signal 632.

According to yet another embodiment, the primary current 630 that flowsthrough the primary winding 560 is sensed by the current sensingresistor 568, which in response outputs a current sensing signal 610 tothe comparators 544, 546 and 548 (e.g., through the LEB component 550).For example, if the switch 566 is closed (e.g., being turned on), thetransformer stores energy and the primary current 622 increases inmagnitude (e.g., linearly), causing the current sensing signal 610(e.g., V_(cs)) to also increase in magnitude (e.g., linearly). Inanother example, the comparator 546 also receives a signal 612 (e.g.,V_(ctrl)) which is generated by the signal conditioning component 536and associated with the amplified signal 608, and outputs a comparisonsignal 636 to the modulation component 538. In yet another example, thecomparator 544 also receives a threshold signal 616 (e.g., V_(th) _(—)_(max)) and outputs a comparison signal 638 to the modulation component538. In yet another example, the comparator 548 receives anotherthreshold signal 618 (e.g., V_(th) _(—) _(min) which is smaller thanV_(th) _(—) _(max) in magnitude) and outputs a comparison signal 640 tothe modulation component 538.

As shown in FIG. 4( a), the line sensing component 530 outputs a signal626 which is associated with an input signal 642 (e.g., V_(in)) incertain embodiments. For example, the voltage-mode component 504receives the signals 612, 626 and 632 and outputs a signal 644 which isreceived by the modulation component 538. In yet another example, themodulation component 538 outputs a modulation signal 646 to the logiccomponent 540 which outputs the signal 632 to close (e.g., to turn on)or to open (e.g., to turn off) the switch 566 in order to affect theprimary current 630. In yet another example, the signal 626 isproportional to the signal 642 in magnitude.

FIG. 4( b) is a simplified diagram showing certain components of thepower conversion system 500 according to an embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. As shown inFIG. 4( b), the system controller 502 further includes a low pass filter708 and a capacitor 710. The voltage-mode component 504 includes asignal generator 702, comparators 704 and 706, and a low pass filter708. For example, the signal conditioning component 536 includes thecapacitor 710. In another example, the switch 566 is a bipolar junctiontransistor.

As shown in FIG. 4( a) and FIG. 4( b), the amplified signal 608 isattenuated and filtered by a compensation network including theresistors 532 and 534, the capacitor 710 and the low pass filter 708,and a filtered signal 712 is received by the comparator 704 (e.g., at anon-inverting terminal), in some embodiments. For example, the signalgenerator 702 receives the signals 626 and 632 and outputs a signal 714(e.g., V_(ramp1)) to the comparators 704 and 706. In another example,the comparator 704 outputs a signal 716 to the modulation component 538,and the comparator 706 which also receives a threshold signal 720 (e.g.,V_min) outputs a signal 718 to the modulation component 538. In yetanother example, the signal 644 is generated by an OR gate that receivesthe signals 716 and 718 as inputs. The signal 714 is a ramping signalassociated with a ramping period which includes a ramping-up period, aramping-down period, and an off period in some embodiments. For example,during the ramping-up period, the signal 714 increases in magnitude;during the ramping-down period, the signal 714 decreases in magnitude;and during the off period, the signal 714 keeps at a low magnitude(e.g., zero).

According to one embodiment, a switching period of the switch 566includes an on-time period during which the switch 566 is closed (e.g.,being turned on) and an off-time period during which the switch 566 isopen (e.g., being turned off). For example, the duration of the on-timeperiod in each switching period and peak values of the primary current630 are affected by the signal 646 generated from the modulationcomponent 538, and thus are affected by the comparison of the signal 712and the signal 714. For example, the signal 714 is a ramping signalwhich increases in magnitude at a slope P in each switching period, andthe slope P of the signal 714 changes with the signal 626. In anotherexample, the slope P increases as the signal 626 increases in magnitude,while the slope P decreases as the signal 626 decreases in magnitude. Inyet another example, the signal 714 is triggered in response to thesignal 632 during each switching period. In yet another example, thesignal 714 begins to increase in magnitude when the signal 632 changesfrom a logic low level to a logic high level.

According to another embodiment, the off-time period in each switchingperiod is used to adjust switching frequency associated with theswitching period. For example, the duration of the off-time period ineach switching period is affected by the comparison signal 628 and thusis affected by the comparison of the signal 608 and the signal 680(e.g., V_(ramp)) generated by the exponential generator 508. In anotherexample, the exponential generator 508 includes a switch-capacitorcircuit that is affected by the clock signal 624 generated by theoscillator 528 (e.g., with a fixed frequency). In yet another example,the signal 680 is determined according to the following equation:

$\begin{matrix}{{V_{ramp}(n)} = {{\left( {V_{refb} - V_{refa}} \right) \times ^{\frac{nT}{\tau}}} + V_{refa}}} & \left( {{Equation}\mspace{14mu} 6} \right)\end{matrix}$

where V_(refb) represents an upper limit of the signal 608, V_(refa)represents a lower limit of the signal 608, T represents a clock periodof the clock signal 624 corresponding to the fixed frequency of theoscillator 528, n represents the number of the clock period, and τrepresents a time constant. As an example, τ is determined according tothe following equation.

$\begin{matrix}{\tau = \left\{ \begin{matrix}{128T} & {0 \leq n \leq 64} \\{256T} & {64 \leq n \leq 128} \\{512T} & {128 \leq n \leq 256} \\{1024T} & {256 \leq n \leq 512}\end{matrix} \right.} & \left( {{Equation}\mspace{14mu} 7} \right)\end{matrix}$

As discussed above and further emphasized here, FIG. 4( a) and FIG. 4(b) are merely examples, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. For example, the switch 566is an IGBT. In another example, the switch 566 is a MOSFET, as shown inFIG. 4( c).

FIG. 4( c) is a simplified diagram showing a power conversion systemthat adjusts switching frequency and peak current in response to outputcurrent according to yet another embodiment of the present invention.This diagram is merely an example, which should not unduly limit thescope of the claims. One of ordinary skill in the art would recognizemany variations, alternatives, and modifications. As shown in FIG. 4(c), the switch 566 is a MOSFET. According to one embodiment, the logiccomponent 540 generates the signal 632 to the driving component 542which outputs a voltage signal (e.g., the signal 699) in order to close(e.g., to turn on) or open (e.g., to turn off) the switch 566.

As discussed above and further emphasized here, FIG. 4( c) is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. For example, the switch 566 is an IGBT instead of aMOSFET.

Also, as discussed above and further emphasized here, FIGS. 3, 4(a),4(b) and 4(c) are merely examples, which should not unduly limit thescope of the claims. One of ordinary skill in the art would recognizemany variations, alternatives, and modifications. For example, aresistor is added to couple between the LEB component 350 and thecomparator 346. In another example, a resistor is added to couplebetween the LEB component 550 and the comparator 546.

FIG. 5 is a simplified timing diagram for the power conversion system500 according to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. As shown in FIG. 5, thewaveform 802 represents the signal 699 (e.g., DRV) as a function oftime, the waveform 804 represents the signal 680 (e.g., V_(ramp)) as afunction of time, the waveform 806 represents the amplified signal 608(e.g., V_(ea)) as a function of time, and the waveform 808 representsthe feedback signal 604 as a function of time. Additionally, thewaveform 810 represents the current sensing signal 610 (e.g., V_(cs)) asa function of time, the waveform 812 represents the threshold signal 616(e.g., V_(th) _(—) _(max)) as a function of time, and the waveform 814represents the signal 612 (e.g., V_(ctrl)) as a function of time. Forexample, when the signal 699 (e.g., DRV) is at a logic high level (e.g.,as shown by the waveform 802), the switch 566 is closed (e.g., beingturned on). In another example, when the signal 699 (e.g., DRV) is at alogic low level (e.g., as shown by the waveform 802), the switch 566 isopen (e.g., being turned off). In yet another example, the waveform 802is substantially the same as the waveform of the signal 632.

As shown in FIG. 5, when the switch 566 is closed (e.g., being turnedon), the transformer including the primary winding 560 and the secondarywinding 562 stores energy, and the primary current 630 increases inmagnitude (e.g., linearly), according to one embodiment. For example,the current sensing signal 610 increases in magnitude, and when thesignal 610 reaches a limit (e.g., the signal 612 or the threshold signal616), the switch 566 is caused to be open (e.g., being turned off). Inyet another example, if the signal 612 (e.g., V_(ctrl)) is larger thanthe threshold signal 618 (e.g., V_(th) _(—) _(min)) but smaller than thethreshold signal 616 (e.g., V_(th) _(—) _(max)) in magnitude, the peakmagnitude of the current sensing signal 610 (e.g., V_(cs) correspondingthe waveform 810) is limited to the magnitude of the signal 612 (e.g.,V_(ctrl) corresponding to the waveform 814).

According to another embodiment, when the switch 566 is open, thetransformer that includes the primary winding 560 and the secondarywinding 562 outputs energy to the output terminal. For example, thedemagnetization process begins (e.g., at time t₁), and the secondarycurrent 694 that flows through the secondary winding 562 decreases inmagnitude (e.g., linearly). The signal 680 (e.g., V_(ramp) correspondingto the waveform 804) is restored to an initial value (e.g., V_(refb)),but after the demagnetization process is completed (e.g., at time t₂),the signal 680 decreases exponentially in one embodiment. For example,if the signal 680 becomes smaller than the amplified signal 608 (e.g.,V_(ea) corresponding to the waveform 806) in magnitude, the comparator506 changes the comparison signal 628 in order to cause the switch 566to be turned on. In another example, the signal 608 (e.g., V_(ea)) islarger in magnitude at heavy load conditions, and the duration of theoff-time period associated with the switch 566 is shorter. In yetanother example, the signal 608 (e.g., V_(ea)) is smaller in magnitudeat light load conditions, and the duration of the off-time periodassociated with the switch 566 is longer which results in a lowerswitching frequency. Referring back to FIG. 2( a), the switchingfrequency has a lower limit (e.g., f_(min)) and an upper limit (e.g.,f_(max)) in some embodiments. For example, at no load or light loadconditions, the switching frequency is fixed at the lower limit (e.g.,f_(min)).

FIG. 6 is a simplified diagram showing certain components of the powerconversion system 500 including the voltage-mode component 504 accordingto an embodiment of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. The voltage-mode component 504 includes a currentsource 738, a capacitor 730, a transistor 732, comparators 704 and 706,and OR gates 734 and 745. For example, the transistor 732 is anN-channel field effect transistor. In another example, the signalgenerator 702 includes the current source 738, the capacitor 730 and thetransistor 732. In yet another example, the LEB component 550 includes aresistor 752 and a transistor 756.

According to one embodiment, during the on-time period of a switchingperiod associated with the switch 566, the transistor 732 is turned offin response to the signal 736, and the capacitor 730 is charged inresponse to the signal 626. For example, the signal 714 (e.g.,V_(ramp1)) increases in magnitude linearly at a slope P. The slope P maybe determined according to the following equation, as an example:

$\begin{matrix}{P = \frac{I\_ vin}{C_{2}}} & \left( {{Equation}\mspace{14mu} 8} \right)\end{matrix}$

where I_vin represents the signal 626, and C₂ represents the capacitanceof the capacitor 730. In another example, the signal 626 changes withthe input line voltage and thus the slope P changes with the input linevoltage.

According to another embodiment, the signal 714 is received by thecomparators 704 and 706 which outputs signals 716 and 718 respectively.For example, the OR gate 734 receives the signals 716 and 718 andoutputs a signal 747 to the OR gate 745. In another example, the OR gate745 receives a control signal 782 (e.g., LEB_b) and outputs a signal 744to the modulation component 538 in order to affect the duration of theon-time period associated with the switch 566. In yet another example,if the signal 712 (e.g., V_(ctrl1)) is larger than the threshold signal720 (e.g., V_min) in magnitude, the duration of the on-time period isdetermined by the signal 712 (e.g., V_(ctrl1)). In yet another example,if the signal 712 (e.g., V_(ctrl1)) is smaller than the threshold signal720 (e.g., V_min) in magnitude, the duration of the on-time period isdetermined by the signal 720 (e.g., V_min). In yet another example,during the off-time period of the switching period associated with theswitch 566, the transistor 732 is turned on in response to the signal736, and the capacitor 730 is discharged. In yet another example, thesignal 714 (e.g., V_(ramp1)) decreases to a low magnitude (e.g., zero).In yet another example, the signal 744 is the same as the signal 644.

According to yet another embodiment, the LEB component 550 that includesthe resistor 752 and the transistor 756 is affected by a control signal780 (e.g., LEB), and outputs the current sensing signal 610 to thecomparator 546. For example, the comparator 546 outputs a comparisonsignal 784 to an OR gate 750 which also receives the control signal 780(e.g., LEB). In another example, the OR gate 750 outputs a signal 786 tothe modulation component 538 in order to affect the status of the switch566. In yet another example, if the control signal 780 is at the logichigh level, the control signal 782 is at the logic low level, and if thecontrol signal 780 is at the logic low level, the control signal 782 isat the logic high level. In yet another example, the control signal 780(e.g., LEB) is an input signal of the LEB component 550. In yet anotherexample, the control signal 780 (e.g., LEB) and the control signal 782(e.g., LEB_b) are associated with a blanking time period during whichthe leading edge blanking is carried out. In yet another example, duringthe blanking time period, the control signal 780 (e.g., LEB) is at thelogic high level and the control signal 782 (e.g., LEB_b) is at thelogic low level. In yet another example, the modulation component 538outputs the signal 646 to the logic component 540 which receives thesignal 628 and outputs the signal 632 (e.g., DR1). The signal 632 (e.g.,DR1) is used as shown in FIG. 4( a), FIG. 4( b) and FIG. 4( c) accordingto certain embodiments.

As discussed above, the slope P of the signal 626 affects the durationof the on-time period of a switching period. For example, the durationof the on-time period corresponds to the pulse width of the signal 699(or the signal 499). In another example, the pulse width of the signal699 (or the signal 499) increases if the slope P decreases, and thepulse width of the signal 699 (or the signal 499) decreases if the slopeP increases. In yet another example, the pulse width of the signal 699increases if the input voltage 642 decreases and if the output voltage602 and the output current 694 remain constant. In yet another example,the pulse width of the signal 699 decreases if the input voltage 642increases and if the output voltage 602 and the output current 694remain constant.

As discussed above and further emphasized here, FIG. 6 is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. For example, the OR gate 734, the OR gate 745 and theOR gate 750 are included in the modulation component 538. In anotherexample, the resistor 754 is removed so that the terminal 610 isdirectly coupled with the resistor 752 and the transistor 756.

FIG. 7( a) is a simplified timing diagram for the voltage-mode component504 as part of the power conversion system 500 if the signal 712 islarger than the threshold signal 720 in magnitude according to anembodiment of the present invention, and FIG. 7( b) is a simplifiedtiming diagram for the voltage-mode component 504 as part of the powerconversion system 500 if the signal 712 is smaller than the thresholdsignal 720 in magnitude according to another embodiment of the presentinvention. These diagrams are merely examples, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications.

As shown in FIG. 7( a), the waveform 902 represents the signal 699(e.g., DRV) as a function of time, the waveform 904 represents thesignal 680 (e.g., V_(ramp)) as a function of time, and the waveform 906represents the amplified signal 608 (e.g., V_(ea)) as a function oftime. Additionally, the waveform 908 represents the signal 712 (e.g.,V_(ctrl1)) as a function of time, the waveform 910 represents the signal714 (e.g., V_(ramp1)) as a function of time, and the waveform 912represents the threshold signal 720 (e.g., V_min) as a function of time.For example, when the signal 699 (e.g., DRV) is at a logic high level(e.g., as shown by the waveform 902), the switch 566 is closed (e.g.,being turned on). In another example, when the signal 699 (e.g., DRV) isat a logic low level (e.g., as shown by the waveform 902), the switch566 is open (e.g., being turned off). In yet another example, thewaveform 902 is substantially the same as the waveform of the signal632.

When the switch 566 is closed (e.g., being turned on), the transformerincluding the primary winding 560 and the secondary winding 562 storesenergy, and the primary current 630 increases in magnitude (e.g.,linearly), according to one embodiment. For example, the transistor 732is turned off in response to the signal 736, and the capacitor 730 ischarged in response to the signal 626. In another example, the signal714 (e.g., V_(ramp1)) increases in magnitude (e.g., linearly) as shownby the waveform 910. Because the signal 712 (e.g., V_(ctrl1)) is largerthan the threshold signal 720 (e.g., V_min) in magnitude, when thesignal 714 becomes approximately equal to the signal 712 (e.g.,V_(ctrl1)) in magnitude, the comparator 706 changes the signal 718 inorder to cause the switch 566 to be opened (e.g., to be turned off), insome embodiments. For example, the duration of the on-time periodincreases with the magnitude of the signal 712 (e.g., V_(ctrl1)).

When the switch 566 is open (e.g., being turned off), the transformerthat includes the primary winding 560 and the secondary winding 562outputs energy to the output terminal according to another embodiment.For example, the demagnetization process begins (e.g., at time t₄), andthe secondary current 694 that flows through the secondary winding 562decreases in magnitude (e.g., linearly). In another example, the signal680 (e.g., V_(ramp) corresponding to the waveform 904) is restored to aninitial value (e.g., V_(refb)), but after the demagnetization process iscompleted (e.g., at time t₅), the signal 680 decreases exponentially asshown by the waveform 904. In yet another example, when the switch 566is open (e.g., at t₄), the transistor 732 is turned on in response tothe signal 736, and the capacitor 730 is discharged. In yet anotherexample, the signal 714 (e.g., V_(ramp1)) decreases to a low magnitude(e.g., zero) as shown by the waveform 910.

As shown in FIG. 7( b), the waveform 1002 represents the signal 699(e.g., DRV) as a function of time, the waveform 1004 represents thesignal 680 (e.g., V_(ramp)) as a function of time, and the waveform 1006represents the amplified signal 608 (e.g., V_(ea)) as a function oftime. Additionally, the waveform 1008 represents the signal 712 (e.g.,V_(ctrl1)) as a function of time, the waveform 1010 represents thesignal 714 (e.g., V_(ramp1)) as a function of time, and the waveform1012 represents the threshold signal 720 (e.g., V_min) as a function oftime. For example, when the signal 699 (e.g., DRV) is at a logic highlevel (e.g., as shown by the waveform 1002), the switch 566 is closed(e.g., being turned on). In another example, when the signal 699 (e.g.,DRV) is at a logic low level (e.g., as shown by the waveform 1002), theswitch 566 is open (e.g., being turned off). In yet another example, thewaveform 1002 is substantially the same as the waveform of the signal632.

When the switch 566 is closed (e.g., being turned on), the transformerincluding the primary winding 560 and the secondary winding 562 storesenergy, and the primary current 630 increases in magnitude (e.g.,linearly), according to one embodiment. For example, the transistor 732is turned off in response to the signal 736, and the capacitor 730 ischarged in response to the signal 626. In another example, the signal714 (e.g., V_(ramp1)) increases in magnitude (e.g., linearly) as shownby the waveform 1010. Because the signal 712 (e.g., V_(ctrl1)) issmaller than the threshold signal 720 (e.g., V_min) in magnitude, whenthe signal 714 becomes approximately equal to the signal 720 (e.g.,V_min) in magnitude, the comparator 704 changes the signal 716 in orderto cause the switch 566 to be opened (e.g., to be turned off), in someembodiments. For example, the duration of the on-time period increaseswith the magnitude of the signal 720 (e.g., V_min).

When the switch 566 is open (e.g., being turned off), the transformerthat includes the primary winding 560 and the secondary winding 562outputs energy to the output terminal according to another embodiment.For example, the demagnetization process begins (e.g., at time t₈), andthe secondary current 694 that flows through the secondary winding 562decreases in magnitude (e.g., linearly). In another example, the signal680 (e.g., V_(ramp) corresponding to the waveform 1004) is restored toan initial value (e.g., V_(refb)), but after the demagnetization processis completed (e.g., at time t₉), the signal 680 decreases exponentiallyas shown by the waveform 1004. In yet another example, when the switch566 is open (e.g., at t₈), the transistor 732 is turned on in responseto the signal 736, and the capacitor 730 is discharged. In yet anotherexample, the signal 714 (e.g., V_(ramp1)) decreases to a low magnitude(e.g., zero) as shown by the waveform 1010.

As discussed above and further emphasized here, FIGS. 3 and 6 are merelyexamples, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. For example, the power conversion system 300 includesall the components as shown in FIG. 6. In another example, thevoltage-mode component 304 operates in the same manner as thevoltage-mode component 504 as shown in FIGS. 7( a) and (b). In oneembodiment, the voltage-mode component 304 includes the OR gate 734, andoutputs the signal 444 based on at least information associated with thesignals 712 and 720 without directly comparing the signals 712 and 720.In another embodiment, the voltage-mode component 304 is configured to,without directly comparing the signals 712 and 720, if the signal 712(e.g., V_(ctrl1)) is larger than the signal 720 (e.g., V_min) inmagnitude and the signal 714 (e.g., V_(ramp1)) is smaller than thesignal 712 (e.g., V_(ctrl1)), generate the signal 444 in order to closethe switch, and if the signal 712 (e.g., V_(ctrl1)) is smaller than thesignal 720 (e.g., V_min) in magnitude and the signal 714 (e.g.,V_(ramp1)) is smaller than the signal 720 (e.g., V_min), generate thesignal 444 in order to close the switch.

According to another embodiment, a system controller for regulating anoutput of a power conversion system includes a signal generator and amodulation and drive component. The signal generator is configured toreceive at least a first signal indicating a magnitude of an inputvoltage received by a primary winding of a power conversion system andreceive a second signal indicating a magnitude of a primary currentflowing through the primary winding, and generate a third signal. Themodulation and drive component is configured to receive at least thethird signal, generate a drive signal based on at least informationassociated with the third signal, and output the drive signal to aswitch to affect the primary current. The signal generator and themodulation and drive component are further configured to, if an outputvoltage of the power conversion system is constant and an output currentof the power conversion system falls within a first predetermined range,generate a modulation signal as the drive signal based on at leastinformation associated with the magnitude of the input voltage withouttaking into account the magnitude of the primary current flowing throughthe primary winding, and if the output voltage is constant and theoutput current falls within a second predetermined range, generate themodulation signal as the drive signal based on at least informationassociated with the magnitude of the primary current without taking intoaccount the magnitude of the input voltage. For example, the systemcontroller is implemented according to at least FIG. 2( a), FIG. 2( b),FIG. 3, FIG. 4( a), FIG. 4( b), and/or FIG. 4( c).

According to yet another embodiment, a system controller for regulatingan output of a power conversion system includes a signal generator and amodulation and drive component. The signal generator is configured toreceive at least a first signal indicating a magnitude of an inputvoltage received by a primary winding of a power conversion system,process information associated with the first signal, and generate asecond signal based on at least information associated with the firstsignal. The modulation and drive component is configured to receive atleast the second signal, generate a drive signal based on at leastinformation associated with the second signal, and output the drivesignal to a switch to affect a primary current flowing through theprimary winding. The signal generator and the modulation and drivecomponent are further configured to, if an output voltage of the powerconversion system is constant and an output current of the powerconversion system falls within a first predetermined range, generate apulse-width-modulation signal corresponding to a pulse width and amodulation frequency as the drive signal. The pulse width decreases ifthe input voltage increases and if the output voltage and the outputcurrent remain constant. For example, the system controller isimplemented according to at least FIG. 2( a), FIG. 2( b), FIG. 3, FIG.4( a), FIG. 4( b), and/or FIG. 4( c).

According to yet another embodiment, a system controller for regulatingan output of a power conversion system includes a signal generator, afirst comparator, a second comparator, and a modulation and drivecomponent. The signal generator is configured to receive at least afirst signal indicating a magnitude of an input voltage received by aprimary winding of a power conversion system, process informationassociated with the first signal, and generate a second signal based onat least information associated with the first signal. The firstcomparator is configured to receive the second signal and a third signalassociated with a feedback signal of the power conversion system andgenerate a first comparison signal based on at least informationassociated with the second signal and the third signal. The secondcomparator is configured to receive the second signal and a thresholdsignal and generate a second comparison signal based on at leastinformation associated with the second signal and the threshold signal.The modulation and drive component is configured to receive at least thefirst comparison signal and the second comparison signal, generate adrive signal based on at least information associated with the firstcomparison signal and the second comparison signal, and output the drivesignal to a switch to affect a primary current flowing through theprimary winding. The modulation and drive component is furtherconfigured to, if the third signal is larger than the threshold signalin magnitude, output the drive signal to close the switch if the secondsignal is smaller than the third signal, and if the threshold signal islarger than the third signal in magnitude, output the drive signal toclose the switch if the second signal is smaller than the thresholdsignal. For example, the system controller is implemented according toat least FIG. 4( a), FIG. 4( b), FIG. 4( c), FIG. 5, FIG. 6, FIG. 7( a)and/or FIG. 7( b).

In one embodiment, a method for regulating an output of a powerconversion system includes receiving at least a first signal indicatinga magnitude of an input voltage received by a primary winding of a powerconversion system, receiving a second signal indicating a magnitude of aprimary current flowing through the primary winding, and processinginformation associated with the first signal and the second signal. Themethod further includes generating a third signal, receiving at leastthe third signal, and processing information associated with the thirdsignal. In addition, the method includes generating a drive signal basedon at least information associated with the third signal, and outputtingthe drive signal to a switch to affect the primary current. The processfor generating a drive signal based on at least information associatedwith the third signal includes, if an output voltage of the powerconversion system is constant and an output current of the powerconversion system falls within a first predetermined range, generating amodulation signal as the drive signal based on at least informationassociated with the magnitude of the input voltage without taking intoaccount the magnitude of the primary current flowing through the primarywinding, and if the output voltage is constant and the output currentfalls within a second predetermined range, generating the modulationsignal as the drive signal based on at least information associated withthe magnitude of the primary current without taking into account themagnitude of the input voltage. For example, the method is implementedaccording to at least FIG. 2( a), FIG. 2( b), FIG. 3, FIG. 4( a), FIG.4( b), and/or FIG. 4( c).

In another embodiment, a method for regulating an output of a powerconversion system includes receiving at least a first signal indicatinga magnitude of an input voltage received by a primary winding of a powerconversion system, processing information associated with the firstsignal, and generating a second signal based on at least informationassociated with the first signal. The method further includes receivingat least the second signal, processing information associated with thesecond signal, generating a drive signal based on at least informationassociated with the second signal, and outputting the drive signal to aswitch to affect a primary current flowing through the primary winding.The process for generating a drive signal based on at least informationassociated with the second signal includes, if an output voltage of thepower conversion system is constant and an output current of the powerconversion system falls within a first predetermined range, generating apulse-width-modulation signal corresponding to a pulse width and amodulation frequency as the drive signal. The pulse width decreases ifthe input voltage increases and if the output voltage and the outputcurrent remain constant. For example, the method is implementedaccording to at least FIG. 2( a), FIG. 2( b), FIG. 3, FIG. 4( a), FIG.4( b), and/or FIG. 4( c).

In yet another embodiment, a method for regulating an output of a powerconversion system includes receiving at least a first signal indicatinga magnitude of an input voltage received by a primary winding of a powerconversion system, processing information associated with the firstsignal, and generating a second signal based on at least informationassociated with the first signal. The method further includes receivingthe second signal and a third signal associated with a feedback signalof the power conversion system, processing information associated withthe second signal and the third signal, and generating a firstcomparison signal based on at least information associated with thesecond signal and the third signal. In addition, the method includesreceiving the second signal and a threshold signal, processinginformation associated with the second signal and the threshold signal,and generating a second comparison signal based on at least informationassociated with the second signal and the threshold signal. Moreover,the method includes receiving at least the first comparison signal andthe second comparison signal, processing information associated with thefirst comparison signal and the second comparison signal, generating adrive signal based on at least information associated with the firstcomparison signal and the second comparison signal, and outputting thedrive signal to a switch to affect a primary current flowing through theprimary winding. The process for outputting the drive signal to a switchto affect a primary current flowing through the primary windingincludes, if the third signal is larger than the threshold signal inmagnitude, outputting the drive signal to close the switch if the secondsignal is smaller than the third signal, and if the threshold signal islarger than the third signal in magnitude, outputting the drive signalto close the switch if the second signal is smaller than the thresholdsignal. For example, the method is implemented according to at leastFIG. 4( a), FIG. 4( b), FIG. 4( c), FIG. 5, FIG. 6, FIG. 7( a) and/orFIG. 7( b).

For example, some or all components of various embodiments of thepresent invention each are, individually and/or in combination with atleast another component, implemented using one or more softwarecomponents, one or more hardware components, and/or one or morecombinations of software and hardware components. In another example,some or all components of various embodiments of the present inventioneach are, individually and/or in combination with at least anothercomponent, implemented in one or more circuits, such as one or moreanalog circuits and/or one or more digital circuits. In yet anotherexample, various embodiments and/or examples of the present inventioncan be combined.

Although specific embodiments of the present invention have beendescribed, it will be understood by those of skill in the art that thereare other embodiments that are equivalent to the described embodiments.Accordingly, it is to be understood that the invention is not to belimited by the specific illustrated embodiments, but only by the scopeof the appended claims.

What is claimed is:
 1. A system controller for regulating an output of apower conversion system, the system controller comprising: a signalgenerator configured to receive at least a first signal indicating amagnitude of an input voltage received by a primary winding of a powerconversion system and receive a second signal indicating a magnitude ofa primary current flowing through the primary winding, and generate athird signal; and a modulation and drive component configured to receiveat least the third signal, generate a drive signal based on at leastinformation associated with the third signal, and output the drivesignal to a switch to affect the primary current; wherein the signalgenerator and the modulation and drive component are further configuredto: if an output voltage of the power conversion system is regulated ina constant-voltage mode and an output current of the power conversionsystem falls within a first predetermined range, generate a modulationsignal as the drive signal based on at least information associated withthe magnitude of the input voltage without taking into account themagnitude of the primary current flowing through the primary winding;and if the output voltage is regulated in the constant-voltage mode andthe output current falls within a second predetermined range, generatethe modulation signal as the drive signal based on at least informationassociated with the magnitude of the primary current without taking intoaccount the magnitude of the input voltage.
 2. The system controller ofclaim 1 wherein the signal generator is further configured to: if theoutput current is larger than a first threshold and smaller than asecond threshold in magnitude, determine that the output current of thepower conversion system falls within the first predetermined range; andif the output current is larger than the second threshold and smallerthan a third threshold in magnitude, determine that the output currentfalls within the second predetermined range.
 3. The system controller ofclaim 2 wherein: the modulation signal corresponds to a modulationfrequency; and the signal generator and the modulation and drivecomponent are further configured to, if the output current is largerthan the first threshold and smaller than the second threshold inmagnitude, keep the modulation frequency at a predetermined frequencyvalue if the output current increases and if the output voltage and theinput voltage remain constant.
 4. The system controller of claim 3wherein the signal generator and the modulation and drive component arefurther configured to, if the output current is larger than the firstthreshold and smaller than the second threshold in magnitude, output themodulation signal to the switch so that a peak value of the primarycurrent increase if the output current increases and if the outputvoltage and the input voltage remain constant.
 5. The system controllerof claim 4 wherein the signal generator and the modulation and drivecomponent are further configured to, if the output current is largerthan the second threshold and smaller than a fourth threshold inmagnitude, increase the modulation frequency if the output currentincreases and if the output voltage and the input voltage remainconstant, the fourth threshold being smaller than the third threshold inmagnitude.
 6. The system controller of claim 5 wherein the signalgenerator and the modulation and drive component are further configuredto, if the output current is larger than the second threshold andsmaller than the fourth threshold in magnitude, output the modulationsignal to the switch so that the peak value of the primary current doesnot change if the output current increases and if the output voltage andthe input voltage remain constant.
 7. The system controller of claim 6wherein the signal generator and the modulation and drive component arefurther configured to, if the output current is larger than the fourththreshold and smaller than a fifth threshold in magnitude, increase themodulation frequency if the output current increases and if the outputvoltage and the input voltage remain constant, the fifth threshold beingsmaller than the third threshold in magnitude.
 8. The system controllerof claim 7 wherein the signal generator and the modulation and drivecomponent are further configured to, if the output current is largerthan the fourth threshold and smaller than the fifth threshold inmagnitude, output the modulation signal to the switch so that the peakvalue of the primary current increases if the output current increasesand if the output voltage and the input voltage remain constant.
 9. Thesystem controller of claim 8 wherein the signal generator and themodulation and drive component are further configured to, if the outputcurrent is larger than the fifth threshold and smaller than the thirdthreshold in magnitude, increase the modulation frequency if the outputcurrent increases and if the output voltage and the input voltage remainconstant.
 10. The system controller of claim 9 wherein the signalgenerator and the modulation and drive component are further configuredto, if the output current is larger than the fifth threshold and smallerthan the third threshold in magnitude, output the modulation signal tothe switch so that the peak value of the primary current does not changeif the output current increases and if the output voltage and the inputvoltage remain constant.
 11. The system controller of claim 1 whereinthe modulation and drive component includes: a modulation componentconfigured to receive at least the third signal and generate a fourthsignal based on at least information associated with the third signal;and a driving component configured to receive the fourth signal andoutput the drive signal to the switch based on at least informationassociated with the fourth signal.
 12. The system controller of claim 1,and further comprising: an error amplifier configured to receive atleast a feedback signal of the power conversion system and generate anamplified signal based on at least information associated with thefeedback signal; and a signal processing component configured to receivethe amplified signal and output a voltage signal to the signal generatorbased on at least information associated with the amplified signal. 13.The system controller of claim 1 wherein the signal generator includes:a ramp-signal generator configured to receive at least the first signal,process information associated with the first signal, and generate afourth signal based on at least information associated with the firstsignal; a first comparator configured to receive the fourth signal and afifth signal associated with a feedback signal of the power conversionsystem and generate a first comparison signal based on at leastinformation associated with the fourth signal and the fifth signal; anda second comparator configured to receive the fourth signal and athreshold signal and generate a second comparison signal based on atleast information associated with the fourth signal and the thresholdsignal.
 14. The system controller of claim 13 wherein the signalgenerator further includes a signal processor configured to receive thefirst comparison signal and the second comparison signal and generatethe third signal based on at least information associated with the firstcomparison signal and the second comparison signal.
 15. The systemcontroller of claim 13 wherein the fourth signal is a ramp signal, theramp signal including a signal pulse for each switching periodcorresponding to the drive signal.
 16. The system controller of claim 1wherein the modulation and drive component is further configured tooutput the drive signal to a bipolar junction transistor as the switchto affect the primary current.
 17. The system controller of claim 1wherein the modulation and drive component is further configured tooutput the drive signal to a metal-oxide-semiconductor field-effecttransistor as the switch to affect the primary current.
 18. The systemcontroller of claim 1 wherein the modulation and drive component isfurther configured to output the drive signal to an insulated-gatebipolar transistor as the switch to affect the primary current.
 19. Asystem controller for regulating an output of a power conversion system,the system controller comprising: a signal generator configured toreceive at least a first signal indicating a magnitude of an inputvoltage received by a primary winding of a power conversion system,process information associated with the first signal, and generate asecond signal based on at least information associated with the firstsignal; and a modulation and drive component configured to receive atleast the second signal, generate a drive signal based on at leastinformation associated with the second signal, and output the drivesignal to a switch to affect a primary current flowing through theprimary winding; wherein: the signal generator and the modulation anddrive component are further configured to, if an output voltage of thepower conversion system is regulated in a constant-voltage mode and anoutput current of the power conversion system falls within a firstpredetermined range, generate a pulse-width-modulation signalcorresponding to a pulse width and a modulation frequency as the drivesignal; and the pulse width decreases if the input voltage increases andif the output voltage and the output current remain constant.
 20. Thesystem controller of claim 19 wherein the signal generator and themodulation and drive component are further configured to, if the outputvoltage of the power conversion system is regulated in theconstant-voltage mode and the output current of the power conversionsystem falls within the first predetermined range, generate thepulse-width-modulation signal corresponding to the pulse width and themodulation frequency as the drive signal, so that a peak value of theprimary current increases if the output current increases and if theoutput voltage and the input voltage remain constant.
 21. The systemcontroller of claim 20 wherein the signal generator and the modulationand drive component are further configured to, if the output voltage ofthe power conversion system is regulated in the constant-voltage modeand the output current of the power conversion system falls within thefirst predetermined range, generate the pulse-width-modulation signalcorresponding to the pulse width and the modulation frequency as thedrive signal, so that a peak value of the primary current increaseslinearly if the output current increases and if the output voltage andthe input voltage remain constant.
 22. The system controller of claim 20wherein the modulation frequency is constant with respect to the outputcurrent.
 23. The system controller of claim 20 wherein the first signalis proportional to the magnitude of the input voltage.
 24. The systemcontroller of claim 20 wherein the signal generator is furtherconfigured to, if the output current is larger than a first thresholdand smaller than a second threshold in magnitude, determine that theoutput current of the power conversion system falls within the firstpredetermined range.
 25. The system controller of claim 19 wherein thesignal generator and the modulation and drive component are furtherconfigured to, if an output voltage of the power conversion system isregulated in the constant-voltage mode and the output current of thepower conversion system falls within the first predetermined range,generate the pulse-width-modulation signal as the drive signal based onat least information associated with the magnitude of the input voltagewithout taking into account the magnitude of the primary current flowingthrough the primary winding.
 26. The system controller of claim 25wherein the signal generator and the modulation and drive component arefurther configured to, if the output voltage is regulated in theconstant-voltage mode and the output current falls within a secondpredetermined range, generate the pulse-width-modulation signal as thedrive signal based on at least information associated with the magnitudeof the primary current without taking into account the magnitude of theinput voltage.
 27. The system controller of claim 26 wherein the signalgenerator is further configured to, if the output current is larger thanthe second threshold and smaller than a third threshold in magnitude,determine that the output current of the power conversion system fallswithin the second predetermined range.
 28. The system controller ofclaim 19 wherein the signal generator includes: a ramp-signal generatorconfigured to receive at least the first signal, process informationassociated with the first signal, and generate a third signal based onat least information associated with the first signal; a firstcomparator configured to receive the third signal and a fourth signalassociated with a feedback signal of the power conversion system andgenerate a first comparison signal based on at least informationassociated with the third signal and the fourth signal; and a secondcomparator configured to receive the third signal and a threshold signaland generate a second comparison signal based on at least informationassociated with the third signal and the threshold signal.
 29. Thesystem controller of claim 28 wherein the signal generator furtherincludes a signal processor configured to receive the first comparisonsignal and the second comparison signal and generate the second signalbased on at least information associated with the first comparisonsignal and the second comparison signal.
 30. The system controller ofclaim 28 wherein the third signal is a ramp signal, the ramp signalincluding a signal pulse for each switching period corresponding to thedrive signal.
 31. The system controller of claim 19 wherein themodulation and drive component is further configured to output the drivesignal to a bipolar junction transistor as the switch to affect theprimary current.
 32. The system controller of claim 19 wherein themodulation and drive component is further configured to output the drivesignal to a metal-oxide-semiconductor field-effect transistor as theswitch to affect the primary current.
 33. The system controller of claim19 wherein the modulation and drive component is further configured tooutput the drive signal to an insulated-gate bipolar transistor as theswitch to affect the primary current.
 34. A system controller forregulating an output of a power conversion system, the system controllercomprising: a signal generator configured to receive at least a firstsignal indicating a magnitude of an input voltage received by a primarywinding of a power conversion system, process information associatedwith the first signal, and generate a second signal based on at leastinformation associated with the first signal; a first comparatorconfigured to receive the second signal and a third signal associatedwith a feedback signal of the power conversion system and generate afirst comparison signal based on at least information associated withthe second signal and the third signal; a second comparator configuredto receive the second signal and a threshold signal and generate asecond comparison signal based on at least information associated withthe second signal and the threshold signal; a modulation and drivecomponent configured to receive at least the first comparison signal andthe second comparison signal, generate a drive signal based on at leastinformation associated with the first comparison signal and the secondcomparison signal, and output the drive signal to a switch to affect aprimary current flowing through the primary winding; wherein themodulation and drive component is further configured to: if the thirdsignal is larger than the threshold signal in magnitude, output thedrive signal to close the switch if the second signal is smaller thanthe third signal; and if the threshold signal is larger than the thirdsignal in magnitude, output the drive signal to close the switch if thesecond signal is smaller than the threshold signal.
 35. The systemcontroller of claim 34 wherein the second signal is a ramp signal, theramp signal including a signal pulse for each switching periodcorresponding to the drive signal.
 36. The system controller of claim 34wherein the first comparator is further configured to receive the thirdsignal associated with an amplified signal generated by an erroramplifier based on at least information associated with the feedbacksignal of the power conversion system.
 37. The system controller ofclaim 34 wherein: the modulation and drive component includes an OR gateconfigured to receive the first comparison signal and the secondcomparison signal and generate a logic signal; and the modulation anddrive component is further configured to generate the drive signal basedon at least information associated with the logic signal.
 38. The systemcontroller of claim 34 wherein: the modulation and drive component isfurther configured to output the drive signal to the signal generator;and the signal generator is further configured to process informationassociated with the drive signal, and generate the second signal basedon at least information associated with the first signal and the drivesignal.
 39. The system controller of claim 34 wherein the modulation anddrive component includes: a signal processor configured to receive thefirst comparison signal and the second comparison signal and generate afirst processed signal based on at least information associated with thefirst comparison signal and the second comparison signal; a modulationcomponent configured to receive at least the first processed signal andgenerate a modulation signal based on at least information associatedwith the first processed signal; and a driving component configured toreceive the modulation signal and generate the drive signal.
 40. Thesystem controller of claim 39 wherein the signal processor includes: afirst OR gate configured to receive the first comparison signal and thesecond comparison signal and generate a second processed signal based onat least information associated with the first comparison signal and thesecond comparison signal; and a second OR gate configured to receive thesecond processed signal and a first leading-edge-blanking signalassociated with a leading-edge-blanking period and generate the firstprocessed signal based on at least information associated with thesecond processed signal and the first leading-edge-blanking signal. 41.The system controller of claim 40, and further comprising: aleading-edge-blanking component configured to receive a secondleading-edge-blanking signal associated with the leading-edge-blankingperiod and a current sensing signal associated with the primary currentflowing through the primary winding and generate a third processedsignal based on at least information associated with the secondleading-edge-blanking signal and the current sensing signal; and a thirdcomparator configured to receive a fourth signal associated with thefeedback signal of the power conversion system and a fifth signalassociated with the third processed signal and generate a thirdcomparison signal based on at least information associated with thefourth signal and the fifth signal.
 42. The system controller of claim41, and further comprising a third OR gate configured to receive thethird comparison signal and the second leading-edge-blanking signal andoutput a fourth processed signal to the modulation and drive component.43. The system controller of claim 42 wherein during theleading-edge-blanking period, the first leading-edge-blanking signal isat a logic high level and the second leading-edge-blanking signal is ata logic low level.
 44. The system controller of claim 34 wherein thesignal generator includes: a capacitor configured to receive the firstsignal and generate the second signal; and a switching componentconfigured to be closed or open in response to a switching signal;wherein: if the switching component is open, the capacitor is configuredto be charged in response to the first signal; and if the switchingcomponent is closed, the capacitor is configured to be dischargedthrough the switching component.
 45. The system controller of claim 34wherein the modulation and drive component is further configured tooutput the drive signal to a bipolar junction transistor as the switchto affect the primary current.
 46. The system controller of claim 34wherein the modulation and drive component is further configured tooutput the drive signal to a metal-oxide-semiconductor field-effecttransistor as the switch to affect the primary current.
 47. The systemcontroller of claim 34 wherein the modulation and drive component isfurther configured to output the drive signal to an insulated-gatebipolar transistor as the switch to affect the primary current.
 48. Amethod for regulating an output of a power conversion system, the methodcomprising: receiving at least a first signal indicating a magnitude ofan input voltage received by a primary winding of a power conversionsystem; receiving a second signal indicating a magnitude of a primarycurrent flowing through the primary winding; processing informationassociated with the first signal and the second signal; generating athird signal; receiving at least the third signal; processinginformation associated with the third signal; generating a drive signalbased on at least information associated with the third signal; andoutputting the drive signal to a switch to affect the primary current;wherein the process for generating a drive signal based on at leastinformation associated with the third signal includes: if an outputvoltage of the power conversion system is regulated in aconstant-voltage mode and an output current of the power conversionsystem falls within a first predetermined range, generating a modulationsignal as the drive signal based on at least information associated withthe magnitude of the input voltage without taking into account themagnitude of the primary current flowing through the primary winding;and if the output voltage is regulated in the constant-voltage mode andthe output current falls within a second predetermined range, generatingthe modulation signal as the drive signal based on at least informationassociated with the magnitude of the primary current without taking intoaccount the magnitude of the input voltage.
 49. A method for regulatingan output of a power conversion system, the method comprising: receivingat least a first signal indicating a magnitude of an input voltagereceived by a primary winding of a power conversion system; processinginformation associated with the first signal; generating a second signalbased on at least information associated with the first signal;receiving at least the second signal; processing information associatedwith the second signal; generating a drive signal based on at leastinformation associated with the second signal; and outputting the drivesignal to a switch to affect a primary current flowing through theprimary winding; wherein: the process for generating a drive signalbased on at least information associated with the second signalincludes, if an output voltage of the power conversion system isregulated in a constant-voltage mode and an output current of the powerconversion system falls within a first predetermined range, generating apulse-width-modulation signal corresponding to a pulse width and amodulation frequency as the drive signal; and the pulse width decreasesif the input voltage increases and if the output voltage and the outputcurrent remain constant.
 50. A method for regulating an output of apower conversion system, the method comprising: receiving at least afirst signal indicating a magnitude of an input voltage received by aprimary winding of a power conversion system; processing informationassociated with the first signal; generating a second signal based on atleast information associated with the first signal; receiving the secondsignal and a third signal associated with a feedback signal of the powerconversion system; processing information associated with the secondsignal and the third signal; generating a first comparison signal basedon at least information associated with the second signal and the thirdsignal; receiving the second signal and a threshold signal; processinginformation associated with the second signal and the threshold signal;generating a second comparison signal based on at least informationassociated with the second signal and the threshold signal; receiving atleast the first comparison signal and the second comparison signal;processing information associated with the first comparison signal andthe second comparison signal; generating a drive signal based on atleast information associated with the first comparison signal and thesecond comparison signal; and outputting the drive signal to a switch toaffect a primary current flowing through the primary winding; whereinthe process for outputting the drive signal to a switch to affect aprimary current flowing through the primary winding includes: if thethird signal is larger than the threshold signal in magnitude,outputting the drive signal to close the switch if the second signal issmaller than the third signal; and if the threshold signal is largerthan the third signal in magnitude, outputting the drive signal to closethe switch if the second signal is smaller than the threshold signal.